There exists a variety of methods which use gradiometers to determine the magnetic field gradient of a system. Gradiometers comprise a plurality of magnetometers separated by a fixed distance. Magnetometers measure the strength and direction of the ambient magnetic field as well as magnetic fields from other sources, such as ferrous materials. The prior art discloses said methods using gradiometers.
One such method using gradiometers is disclosed in U.S. Pat. No. 5,122,744 to Koch, which is herein incorporated by reference for all that it contains. Koch discloses a gradiometer which utilizes at least three vector magnetometers (preferably SQUIDs) to measure a magnetic field gradient. The gradiometer includes a reference magnetometer and a plurality of sensor magnetometers, wherein the reference magnetometer (SQUID) is used to cancel background magnetic fields from outputs of the sensor magnetometers, via a feedback loop provided with a signal from the reference magnetometer. Similarly, higher order gradiometers can be built using a reference magnetometer cube and a plurality of sensor magnetometer cubes.
Another such method using gradiometers is disclosed in U.S. Pat. No. 5,657,756 to Vrba et al., which is herein incorporated by reference for all that it contains. Vrba et al. discloses a method and system of obtaining magnetic measurements with a preselected order of sensor gradiometer which are substantially equivalent to those which would be obtained with a sensor gradiometer of a higher order. The system and method employs a reference system comprising reference tensor gradiometers and appropriate components of a measured tensor are combined with the measurements obtained from the sensor gradiometer to obtain a magnetic measurement substantially equivalent to that which would have been measured by a higher order sensor gradiometer.